The production of polysilicon chunk materials via the decomposition of a gaseous precursor compound on a slim rod substrate is a well-known, widely used process commonly referred to as the “Siemens process.” The Siemens process is a combined decomposition/deposition process that comprises: (1) one or more slim rods (appropriate substrates) covered by a suitable enclosure to allow high temperature, air-tight operation; (2) a system to feed the precursor material or compound of desired composition without contamination; (3) heating the enclosed slim rods to a desired temperature under appropriate environment; (4) decomposing the precursor material preferentially on the heated surface of the slim rods; (5) recovery or disposal of byproduct or unreactant gases; and (6) recovery of product without contaminating the product.
Typical Siemens processes and reactors, the reactant gas is fed to the slim rods from a single port resulting in uneven growth. Such uneven gas distribution over the length of the rod further promotes heavy homogeneous nucleation. Such uneven growth and homogeneous nucleation promote eventual reactor failure. Moreover, the slim rods within typical Siemens process reactors are not individually isolated. As a result, homogeneous nucleation, lower conversion, higher by-products, and uneven growth on the rods is further promoted by uneven radiant heat between the rods and gas precursor distribution.
Known systems utilizing the Siemens process use at least two power supplies hooked to each reactor connection system. One or more primary power supply is used for heating and maintaining the temperature of the slim rod (or deposition rods) system for gas decomposition/deposition. A secondary power supply is necessary at initiation to overcome the resistance of the silicon rod and must supply very high voltage, greater than about 26,000 volts for a typical reactor (with the voltage based on the length and diameter of the slim rod assembly used). The necessity for a high voltage power supply significantly increases the cost and safety concerns of operating such known reactors.
In some known reactors, rather than use a very high voltage source, a heating finger is introduced into the reaction space and parallel to the deposition rods. To preheat the reaction space, the heating finger is lowered into the reaction space in the proximity of the deposition rods. Once the slim rods are at the optimum eclectically conductive condition and temperature and electrical current can be passed through the rods, the heating finger is removed from the reactor, and the opening in the metallic enclosure is sealed. These reactors present further issues with the purity and integrity of the product, throughput, and establishing and maintaining a seal as well as safety, operational and maintenance issues.
According to known processes, elemental silicon is obtained in the Siemens type reactor, in the form of cylindrical rods of high purity by decomposing silicon halides from the gas phase at a hot surface of the pure and purified silicon filament, the preferred halides being the chlorides, silicon tetrachloride and trichlorosilane. These compounds become increasingly unstable at temperatures above 800° C. and decompose. Heterogeneous nucleation, hence silicon deposition, starts at about 800° C. and extends to the melting point of silicon at 1420° C. Since the deposition is beneficial only on the substrate, the inner walls of the decomposition chamber must not reach temperatures near 800° C. in order to prevent wasteful deposition on the chamber walls. In known Siemens process reactors, the reactor walls are generally cooled to prevent such wasteful deposition and also to maintain the structural integrity of the assembly. However, cooling the walls consumes additional energy to over come the additional temperature difference.
A further issue with the cooling of the reactor walls is the thermophoretic deposition of powder particles on the cooled reactor walls. Such deposition is generally weak resulting in the multiple recirculation of the particles in the gas stream. This deposited powder eventually gets loose and collapse into the reactor, causing premature failure.
The silicon halides used most frequently for the preparation of high purity silicon are silicon tetrachloride and trichlorosilane. These halides will undergo pyrolysis when in contact with the hot surface and deposit elemental silicon. To obtain reasonable and economical yields, however, an excess of hydrogen gas is added to the silicon halide vapor reaction feed gas. Because of its proportionally higher silicon content per unit weight and comparatively lower deposition temperature (i.e., faster kinetics), trichlorosilane will deposit more silicon than silicon tetrachloride and is therefore the preferred material for the Siemens' process for the preparation of polycrystalline silicon. Silicon halides with less than three chlorine atoms, such as SiH2Cl2 and SiH3Cl, in particular, deposit much more silicon per mole of silicon halide consumed in the reaction but are impractical because they are not readily available and thus less desirable economically. In any case, the yield is not more than 20% and by-product gases are very difficult to handle.
Another approach to improved deposition rates is to use mixtures of silane and hydrogen where fast kinetics and lower temperatures assist faster deposition and better conversion. For example, silane (SiH4) offers itself as an effective silicon precursor and having no chlorine in the molecule improves the silicon to hydrogen ratios of silicon reaction gas mixtures. Silane decomposes above 400° C. forming silicon and hydrogen. The byproducts formed are silane and hydrogen which may be readily recycled.